Blue-Green Algae (Cyanobacteria) Blooms

Last Update: September 18, 2013

Activities at the local, state, national and international level demonstrate an increasing interest in health concerns related to blue-green algae (cyanobacteria) and harmful algal blooms. In response, the Environmental Management Branch is providing information and links (at bottom of page) to other sites that will be helpful to the public and to local, regional, and state public health and environmental health officials.

General Information

What are blue-green algae blooms?

Blue-green algae (cyanobacteria) are any of a number of species of microscopic bacteria that are photosynthetic. They may exist as single cells or groups of cells. They occur naturally in surface waters.

When conditions are optimal, including light and temperature, levels of nutrients (i.e., phosphorous and nitrogen, and the ratio of the two), and lack of water turbulence, blue-green algae can quickly multiply into a bloom.

Blue-green algae blooms are likely to occur more often in warmer months.

What is the concern about blue-green algae blooms?

When some blooms occur in water bodies, exposure to the blue-green algae and their toxins can pose risks to humans, pets, livestock and wildlife. Exposure may occur by ingestion, dermal contact, and aspiration or inhalation.

Risks to people may occur when recreating in water in which a blue-green algae bloom is present, or from the use of drinking water that uses a surface water source in which a blue-green algae bloom is present.

What are the possible health effects related to blue-green algae blooms?

Exposure to blue-green algae can cause rashes, skin and eye irritation, allergic reactions, gastrointestinal upset, and other effects. At high levels, exposure can result in serious illness or death.

Depending on the particular cyanobacterium, and the amount to which one is exposed, blue-green algae have the potential to cause a variety of adverse health effects, including liver toxicity (e.g., Microcystis aeruginosa) and neurotoxicity (e.g., Anabaena circinalis). Microcystin toxins may also promote tumor growth.

Destruction of cyanobacteria cells may release the toxins into surrounding waters, so care must be taken in dealing with blue-green algae blooms.

Are these effects just theoretical?

No. Several dog deaths have been reported following the dogs' exposure to blue-green algae in Humboldt County, as mentioned in this press release (PDF)

What is the best way to minimize risks from blue green algae in drinking water?

Avoid drinking untreated surface water. This will protect you not only from blue-green algae, but from Giardia, Cryptosporidium, and a host of other microbes.

What about drinking water from public water systems?

Water systems generally go to great lengths to keep blue-green algae from growing in their surface water supplies, in order to avoid taste and odor problems that would concern their customers. For example, some blue-green algae produce geosmin and MIB (2-methylisoborneol); their earthy, musty odors can be sensed at very low concentrations (on the order of nanograms per liter). The usual good management practices to control taste and odor help reduce the likelihood of toxic blue-green algal blooms.

When toxic blue-green algae blooms occur, water systems need to utilize a strategy to treat the blooms that does not merely kill the algae, since rupturing (or lysing) the blue-green algal cells can release their toxins, and treatment may not be entirely effective in removing toxins.

What is the regulatory status of blue-green algae in drinking water?

There aren't any drinking water standards specifically for blue-green algae, but "blue green algae (cyanobacteria), other freshwater algae, and their toxins" were included on the second federal Drinking Water Contaminant Candidate List (CCL2). EPA uses the CCL to prioritize research and data collection efforts to help determine whether a specific contaminant ought to be regulated.

What is the best way to minimize risks from blue green algae in recreational water?

Children especially should avoid contact. Their small body weight means their exposures to blue green algae will be higher than adults, given the same volume of water intake.

Local health and environmental agencies are encouraged to and generally do post recreational areas with signs and brochures to inform the public of the presence of blue-green algae blooms. Some examples of information from various states are presented below.

What about fish caught in water experiencing a blue-green algae bloom?

If one is going to consume fish, it's best to remove the internal organs, which would likely contain more of the algae/toxin.